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GEOMORPHIC RESTORATION OF UNSTABLE REACH OF ADOBE CREEK FOR BANKFULL TO FLOOD FREQUENCIES BASED ON MATCHING VELOCITIES AND TOTAL WORK ON STREAM BED AND BANK TO STABLE REFERENCE REACHES

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A four step method was developed and applied to restore 750 foot (225 meters) of gunnite encased and eroding section of Adobe Creek. Restoration involved minimizing hardscape, maximizing natural bed and bank features, providing the correct bankfull and top of bank channel slope and shape to improve stability, improve flood conveyance and sediment transport, and improve habitat and fish passage. The four steps included: (1) geomorphic evaluation of stable reference and unstable reaches, which includes both step-pool and riffle-pool geometries; (2) hydrologic modeling and calibration using a continuous simulation model (HEC-HMS) to compute runoff based on 50 years of continuous rainfall data using current land cover; (3) hydraulic modeling (HEC-RAS + additional model), shear stress, and work (energy) calculations at stable and actively eroding cross-sections along the creek based on results of the hydrologic modeling; and (4) computation of work (energy) and velocity for different channel configurations to identify a stable configuration that can be designed within the constraints. Since the excess work is computed only for the flow conditions over time under which the applied shear exceeds the critical shear of the bed or bank material, it is also a means to evaluate sediment transport of the restored cross-section relative to a stable cross-section.

The bankfull channel depth and width were measured on several stable sections upstream of the reach to be restored. The analysis showed that the bankfull flow at this reach, which drains 10 square kilometers of rural + urban watershed, was 85 ft3/sec (2.4 m3/sec). The flow frequency analysis showed that the return interval for this flow was 1 year. The return interval decreases from greater than 2 years in the rural upper reaches to less than 1 year as the watershed goes through urbanization.

The channel velocities were computed for stable reference sections and ranged from 3.3 to 7.2 ft/sec (1 m/sec to 2.2 m/sec) from bankfull flows of 85 cfs (ft3/sec) to flood flows of 1500 cfs. The excess work done per square foot on the stream bed and toes of banks was less than 200 Million ft-lbs (<2900 Million Joules per square meter) on stable reference sections, between 200 and 300 Million ft-lbs (2900 to 4300 Million Joules per square meter) on eroding but still stable sections, and greater than 300 Million ft-lbs (4300 Million Joules per square meter) on actively eroding sections. This information was used to shape the channel slope and cross-section in the restored section to have work done of 150 Million ft-lbs (2200 Million Joules per square meter of bed).

The bankfull channel and the top of bank channel were shaped to avoid designing an incised channel in which excessive shear stress and work could be exerted on the channel bed. For this, every effort was made to insure that the channel width at 2 × bankfull depth was greater than 2 times the bankfull width. The meander wavelength, amplitude and sinuosity were deteremined from the analysis of reference reaches immediately upstream.
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Document Type: Research Article

Publication date: 01 January 2005

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